How do glaciers erode?

When gravity causes ice to move down the side of a mountain there are two ways in which the rock below is eroded. These are plucking and abrasion. Freeze-thaw weathering is a process that also affects glacial environments.

Plucking is when meltwater from a glacier freezes around lumps of cracked and broken rock. When the ice moves downhill, rock is plucked from the back wall. Plucking is particularly effective when the rock contains joints (cracks) the water can seep into. Meltwater is found under a glacier due to the weight of the ice. The great pressure causes ice at the base of the glacier to melt. This is known as pressure melting. Also, meltwater flows from the surface of the glacier to its base through large cracks, known as crevasses, in the ice.

Abrasion is when rock that is frozen to the base and the back of the glacier scrapes the bedrock. This acts like sandpaper and erodes the bedrock. Enormous damage can be done by very large boulders. Large scars created in the bedrock by this process are called striations.

When a large amount of plucking has occurred this increases the rate of abrasion as more rock is embedded in the base of the glacier.

At the top end of the glacier the ice doesn’t move in a straight line — it moves in a circular motion called rotational slip. This can erode hollows in the landscape and deepen them into bowl shapes called corries.

Processes of erosion and weathering

What is the effect of freeze-thaw weathering?

Weathering is the break down of rock in situ (in the place where it is). Freeze-thaw is when meltwater or rain gets into cracks in the bedrock, usually the back wall. At night the water freezes, expands and causes the crack to get larger. Eventually, the rock will break away.

Freeze-thaw weathering

In glaciated areas, freeze-thaw weathering (or frost shattering) takes place on rock surfaces above the surface of the ice and at its margins.

The evidence for freeze-thaw weathering is seen in landscape features called scree slopes and blockfields. These are piles of rock that cover large upland areas in the UK. Some scree slopes and blockfields date from the last ice age, however, others were more recently formed.